"Canning" food products involves the packing of food in rigid, sealed containers, usually metal or glass, under conditions which permit the packaged item to be indefinitely shelf stable at normal room temperatures. Generally, canned food products have exhibited characteristic heat processed flavors and inferior texture and/or color to the consumer. With the advent of improved transportation and distribution of fresh foods, and the widespread use of other food preservation techniques, such as freezing, the inferior flavor, texture and color of canned products have become, by comparison, more noticeable to the consuming public. In some instances this factor is thought to have contributed to consumer rejection of otherwise wholesome and stable canned items.
The heat processed flavors are encountered in both meat and vegetable products. Certain of the inferior flavors result from the food preparation method and are identified as "cooked in the can" or "processed" flavors. These are promoted by at least two conditions encountered in prior art canning operations, namely, overcooking and cooking in a sealed container in the presence of relatively large portions of liquid diluting material which is normally canned with the garnish. These heat processed flavors are distinct from the metallic taste sometimes imparted to food packed in metal cans, the latter being a characteristic of the container rather than the food preparation.
In the usual canning process the cans are filled with the garnish and liquid components, sealed, and retorted either continuously or in large batches. Retorting, while primarily intended to sterilize the product, also cooks it within the sealed can. It cannot be said that all bacteria and spoilage organisms are destroyed during retorting, but commercially acceptable and nutritionally safe sterilization levels may be obtained by holding the product at elevated temperatures for periods of time inversely proportional to the temperature. That is, generally, at higher temperatures, shorter periods of time are lethal for a sufficient percentage of bacteria to effect sterilization. Reference is directed to the National Canners Association "Laboratory Manual for the Canning Industry," 2d edition, 1956, for further information on temperature and time requirements to effect sterilization in the canning process.
Temperatures must be held within practical limits to avoid excessive thermal damage to the product, and to avoid excessive stresses upon the cans. Unfortunately, in the retorting process the time and temperature conditions necessary to effect sterilization also result in over treatment of at least portions of the food product. In this regard it must be noted that heat transfer within the sealed can will result in the outer portions of the product reaching relatively high temperatures for longer periods as contrasted with the inner portions. Consequently, thermal degradation primarily of the liquid component of the product occurs.
Additionally, the heating of a garnish in the presence of large amounts of liquid component, usually water, also appears to impair flavor or taste values of the product primarily due to loss of flavor material to the liquid and to losses associated with chemical reactions. The rates of these chemical reactions increase as the temperature increases. This effect is similar to cooking in excess water or boiling often employed by the homemaker with tough or poor quality foods. Furthermore, when food products are cooked in a closed container or vessel, any undesirable volatile materials which would normally escape in home cooking will be retained within the product, probably in the diluting agent.
The conventional retorting process, in addition to impairing certain flavor values, is also highly energy inefficient, since it requires large batches of canned product to be placed in a retort vessel, which is then raised to a superatmospheric pressure and elevated temperature for a time sufficient for all of the product within each can to reach and hold a given temperature until sterilized. The energy required to accomplish this result is more than that required to sterilize the components individually. Further, the cans must be cooled and the pressure within the retort reduced before the latter may be opened and the cans removed. This requires costly and space consuming cooling equipment.
In an attempt to overcome the disadvantages of retorting, the canning industry has directed much interest to processes for aseptically sterilizing food products before can filling. Aseptic sterilization of foods refers to the sterilization of foods under sterile environmental conditions within a particular field or area, and usually involves techniques for keeping micro-organisms from the area. This contrasts with the above-described retorting method, wherein unsterilized food is placed within an unsterile container in an unsterile environment and is subsequently heated to commercially sterilize the contents. Aseptic methods have been employed primarily with food products containing no suspended particles or garnish, such as fruit juices or milk products. The prior art has described processes in which the garnish and liquid components of the product are separately, aseptically sterilized and are delivered to a sterile container for final packaging. (See, e.g., U.S. Pat. Nos. 3,232,770, 3,241,475, and 3,437,495). Because the garnish is sterilized outside of the ultimate container, considerable handling of the delicate garnish is required after sterilization by heating in order to deliver to the ultimate container. In the case of many types of delicate garnish, it has been found that this handling results in considerable physical damage to the garnish.
In Baker U.S. Pat. No. 273,436, a process for canning green corn on the cob is disclosed which involves placing the corn and a very small quantity of water into a can, sealing the can, heating the can to flash the water into steam, which cooks and sterilizes the corn, opening the can and filling it with brine, and resealing the can and heating the can again. While this process serves to better preserve the tenderness and flavor of the corn itself, inherent in the process is a degradation of the product due to the secondary heating of the product with the brine in the can.